A cannula is basically a small tube made for insertion into a body cavity or into a duct or vessel for infusion, draining and monitoring. Cannulae also include various types of specialized tubing that are used in the surgical field to transport blood from the heart to a cardiopulmonary pump and oxygenator, and to return the blood to the circulatory system. While there are standard configurations of cannulae, many are custom-designed to suit the individual surgeon's requirements. Leading corporations produce more than 1,200 types of cannulae and accessories to facilitate cardiopulmonary bypass. More recent developments include a line of cannulae to facilitate vacuum-assisted venous drainage during cardiopulmonary bypass and dispersion aortic cannulae, which are used to reduce the pressure of blood flow returning to the body through the aorta.
Cannulae and catheters are manufactured by utilizing various different processes. U.S. Pat. Nos. 3,802,908, 5,537,729, 5,722,395, and 5,785,998 describe some of the current manufacturing processes that may be utilized in producing cannulae and catheters and are incorporated herein by reference. Pat. No. 3,802,908 issued to Emmons discloses a process for forming a multi-layer resinous coating by applying flowable heat softened resinous material directly to the external surface of a rotating cylindrical pipe at ambient temperature. U.S. Pat. Nos. 5,537,729, 5,722,395, and 5,785,998 issued to Kolobow disclose a method of making ultra thin walled wire reinforced endotracheal tubing. Kolobow discloses thin walled tubing manufactured of a polymeric material having a spring wire material incorporated therewith. Utilization of the spring wire material in combination with the polymeric material results in reduced wall thickness, which decreases the resistance to airflow through the endotracheal tubing. The endotracheal tubing is made by depositing a dissolvable polymeric material on a rotating mandrel in successive layers. A spring material is also applied around the mandrel to produce the ultra thin walled wire reinforced endotracheal tubing. By controlling the rate of deposition of the polymeric material along the length of the mandrel, different wall thicknesses of tubing may be achieved.
During an extrusion process, polymer material is forced through a die. Almost any solid or hollow cross-section may be produced by utilizing the extrusion process. After a first layer is applied over the mandrel utilizing the extrusion process, a wire is wrapped around the first layer. Once wrapped with the wire, a second layer is extruded over the wire to produce a cannula that is wire-reinforced. Once the plastic reinforced tube is extruded, it is stripped from the mandrel, and cut in a desired length. Additional pieces, depending on the specific design of the cannula, are molded, welded, or connected to the plastic reinforced tube to create a cannula. These various process steps can be very expensive because of small lots of production and varying sizes of cannulae. Most of the cost savings with the extrusion process come from being able to run thousands of feet at a time.
A dipping process is commonly used with silicon and other types of thermal set material. U.S. Pat. No. 5,885,251 issued to Luther discloses a dipping process associated with producing a catheter and is incorporated herein by reference. The catheter is formed by a sequential mandrel dip process. More particularly, a rigid tip portion may be formed by dipping a mandrel into a solubilized softenable material and allowing the softenable material to dry on the mandrel. The softenable material is preferably hydrated from the mandrel and the desired length for the distal tip portion is cut off. This distal portion is then applied to and dried on a secondary dipping mandrel and a length of flexible catheter is inserted over the mandrel such that the catheter abuts the softenable distal portion. The softenable portion and the portion of flexible catheter are then dipped into a liquid polymer such that the two portions are solvent welded together and an outer layer of polymer is deposited thereover. Thus a contiguous assembly is formed by bonding the softenable material to the distal end of the flexible catheter. The assembly is then allowed to dry and the dipped softenable tip portion is trimmed to exhibit a profile that facilitates insertion into a patient. The mandrel is subsequently removed from the assembly of the catheter and the softenable portion such that a continuous lumen is formed within both portions. A needle cannula is then inserted through the wall of the catheter and extended axially through the length of the catheter such that its sharp tip extends outwardly beyond the softenable end of the catheter.
Although the dipping process may be desirable in certain situations, dipping is sometimes not a desirable option for cannulae manufacturing because of excessive cost, extended process cycle time, many secondary steps to make a final product, and a need for on-going monitoring and adjustments during the manufacturing process. These variables and additional process steps significantly increase the cost of the product.